Terminazioni germaniche negli epiteti delle Matronae: Aflims, Saitchamims, Vatvims Le Matronae sono le divinità più venerate della Germania Inferior, le dediche si concentrano

The vibro-replacement technique uses the vibrator or poker to displace and compact the in situ ground, as in vibro-compaction. But in this case, rather than collapsing the soil, a hole is formed into which stone is placed and compacted to form a load-bearing ‘stone column’. Expert assessment is essential.

The principle is to replace loose material by compacted stone, together with Ground improvement 171

densification and reduction in compressibility of the surrounding ground to form a composite material. Because the stone columns will deform under applied load, the capacity of the columns depends on the degree of stiffening achieved in the surrounding soil as well as on the internal friction of the columns. In fine soils the stone columns will act as drainage paths to accelerate the rate of consolidation—provided that the drained water can be dispersed.

Suitable for reinforcing most loose granular soils, fills, weak and cohesive soils; shear strength of the soil to be treated should be >20kN/m².

Not recommended where peat layers exist, nor in loose collapsible fills with voids.

Obstructions to penetration should be avoided—e.g. rubble in demolished basements. In loose unsaturated fills stone columns may provide a pathway for surface water into underlying untreated soil, possibly causing settlement.

Applications include foundations for low-rise structures, strip footings and light rafts, venting wells on methane-generating landfill, reduction in the liquefaction potential of fine soils (Priebe, 1998) and embankment foundations.

Ground investigation is as required for vibro-compaction (6.3). The chemical composition and levels of contamination of the ground to be treated are important when considering the type of stone to be used for the columns. Penetration tests to determine the initial density of the ground are important.

On complex and large sites a trial treatment with large-scale loading is recommended.

Design method (mainly empirical) will depend on the ground being treated as the interaction of the soil, stone column and foundation is complex.

Ultimate bearing capacity of a single stone column in cohesive soil can be obtained using the Hughes and Withers (1974) method:

where is the internal angle of friction, c_u undrained shear strength of the clay, γ the Suaitable soils for vibro-replacement

An introduction to geotechnical processes 172

effective unit weight, z the depth, q any surcharge and u the pore pressure. The ratio of column length to diameter is critical to bearing capacity and should be <6 in cohesive soil.

Settlement of a single stone column can be estimated from Bauman and Bauer (1974).

However, because of the variation in the actual size of columns compared with the assumptions, neither of the above mathematical approaches is particularly reliable.

Simple, but conservative, design charts (Moseley and Priebe, 1993), expanded by Priebe (1995), consider load distribution and lateral support from the combined stone column-stiffened surrounding ground on an area basis to give an ‘improvement factor’ n.

The improvement factor indicates the increase in compression modulus and the extent to which the settlement will be reduced by the column/ ground improvement. This is compared with the area ratio (the ratio of the area being treated by each column A—based on the column centres—over the stone column plan area A_c). In soft compressible soils, the stone columns are likely to bulge and settle and transfer applied load to the clay; if no bulging occurs the column carries all the load on to the bearing layer.

The BRE Specification (Watts, 2000) also provides information on design and construction.

Depth of treatment can be obtained directly from the Priebe graphs. Where possible the stone columns should penetrate fully to a sound bearing layer. The type of rig and placement method for the stone will limit the depth possible—around 10m for bottom-feed rigs.

The top 1m of the treated ground will be less well compacted and will require some additional rolling before placing shallow foundations.

Long stone columns under concentrated loads are not advisable as the load is dissipated into the ground at a relatively high level as the column bulges.

Layout of columns will depend on the foundation being supported:

• a single line of stone columns under strip footings

Improvement in settlement with stone colums Ground improvement 173

• a triangular grid for areal treatment—at around 1–3m centres depending on the diameter of column which can be formed and subject to remaining within the Priebe (1995) parameters.

The strength of treated ground gives allowable bearing pressures for foundation design of 150–400kN/m² depending on the type of ground treated, column spacing and whether the columns are fully or partially penetrating to a bearing layer.

The long-term performance of the stone column may be affected by deterioration of the stone aggregate and reduction in the support of the surrounding ground.

Construction uses similar plant to that for vibro-compaction (6.3), but here the stone used is inert, crushed, graded aggregate normally in the range 40–100mm. The stone used must be selected so that it will remain stable for the life of the structure being supported—e.g. limestone is not suitable for acidic conditions.

If hard layers exist in the ground to be treated, it may be necessary to pre-drill before inserting the poker to form the columns. The benefit of speed of installation of stone columns is then lost.

The following are methods for stone columns using vibro-replacement.

The top-feed system is used in dry conditions in stiff soils, where the poker penetrates the ground under its own weight using compressed air and vibration. At the required depth the poker is withdrawn and a small charge of the stone is introduced and compacted by the vibrating poker to interlock with the ground. The process is repeated by adding and compacting stone until a dense column of stone is built up to ground level.

The diameter of columns will be limited to the poker diameter in stiff soil.

The wet system is used with top feed of the stone in cohesionless soils below the water table and in weak silts and clays. The vibrating poker penetrates to depth under its own weight, with water jets removing fines. At the required depth the water pressure is reduced to ensure that the hole formed remains open with water circulation and stone is introduced down the annulus and compacted in short lifts to build up the column to the

Stone column parameters

An introduction to geotechnical processes 174

surface. Column diameter will usually be larger than the poker.

The bottom-feed system can be used below the water table without water jetting, thereby avoiding the problems of disposal of the surplus water. In addition to penetration under its self-weight, compressed air and vibration, the rig may provide additional pull-down force on the poker. The poker has a stone supply tube and hopper attached to deliver stone to the tip of the poker assisted by compressed air. At the required depth the poker is withdrawn 1m, and a designed amount of stone is discharged at the tip and then compacted by the surging action of the vibrator. The process is continued until the compacted stone column reaches the surface.

Some ground heave may occur in cohesive soils.

Plant and equipment required is similar to that for vibro-compaction: a 300–450mm diameter poker weighing 3–5 tonnes. Specialist rigs are now common for the dry bottom-feed system.

Cased and rammed stone columns are used in high-sensitivity clays where vibro-techniques are likely to weaken the surrounding soil (Chummar, 1998).

Monitoring using CPTs or SPTs has limited value in verifying the dimensions and internal angle of friction of the column. An improvement in N-value of 10 should be achieved in the surrounding granular soil. Stiffening is unlikely in cohesive soil, causing

‘hard points’ at the columns.

Zone loading tests over a group of columns are the best means of ensuring the settlement and bearing capacity. Construction quality is monitored by the power take-up of the poker and the rate of stone placement.

Stone columns by bottom feed method Ground improvement 175